1. Field of the Invention
This invention relates to a method of monitoring the progress and completion of slag removal in a partial oxidation reactor during controlled oxidation.
2. Description of the Prior Art
Fuels such as petroleum coke, residual fuel oils or other contaminated hydrocarbonaceous materials that undergo partial oxidation in a partial oxidation reactor produce a slag byproduct that can collect and build up deposits on the inside surface of the reactor or the reactor outlet to an amount that prevents effective partial oxidation. Periodic shutdown of the partial oxidation reactor then becomes necessary to remove slag, in an operation commonly referred to as "controlled oxidation" or "deslagging."
The slag-depositing material in the fuel or feedstock of the partial oxidation reactor exists as an impurity or contaminant. The constituency of the slag-depositing material can vary depending upon the feedstock and its source.
The slag-depositing material has a slagging component, which is an element or compound that, alone or in combination with another material in the reactor, such as oxygen or sulfur, forms slag. Slagging elements include transition metals, such as vanadium, iron, nickel, tantalum, tungsten, chromium, manganese, zinc, cadmium, molybdenum, copper, cobalt, platinum, palladium; alkali and alkaline earth metals, such as sodium, potassium, magnesium, calcium, strontium, or barium; and others including aluminum, silicon, phosphorus, germanium, gallium, and the like. The amount of slagging elements in the feedstock generally varies from about 0.01 to about 5 weight %.
A typical charge to a partial oxidation reactor includes the feedstock, a free-oxygen containing gas and any other materials that may enter the burner located in the reactor inlet. The partial oxidation reactor is also referred to as a "partial oxidation gasifier reactor" or simply a "reactor" or "gasifier," and these terms are used interchangeably throughout the specification.
Any effective burner design can be used, although typically a vertical, cylindrically shaped steel pressure vessel with a reaction zone preferably comprising a down-flowing, free-flow refractory lined chamber with a centrally located inlet at the top and an axially aligned outlet at the bottom is preferred.
These reactors are well known in the art, as are the partial oxidation reaction conditions. See, for example, U.S. Pat. Nos. 4,328,006 and 4,328,008, both to Muenger, et al., U.S. Pat. No. 2,928,460 to Eastman, et al., U.S. Pat. No. 2,809,104 to Strasser et al., U.S. Pat. No. 2,818,326 to Eastman et al., U.S. Pat. No. 3,544,291 to Schlinger et al., U.S. Pat. No. 4,637,823 to Dach, U.S. Pat. No. 4,653,677 to Peters et al., U.S. Pat. No. 4,872,886 to Henley et al., U.S. Pat. No. 4,456,546 to Van der Berg, U.S. Pat. No. 4,671,806 to Stil et al. , U.S. Pat. No. 4,760,667 to Eckstein et al., U.S. Pat. No. 4,146,370 to van Herwijner et al., U.S. Pat. No. 4,823,741 to Davis et al., U.S. Pat. No. 4,889,540 to Segerstrom et al., U.S. Pat. Nos. 4,959,080 and 4,979,964, both to Sternling, and U.S. Pat. No. 5,281,243 to Leininger.
The partial oxidation reaction is conducted under reaction conditions that are sufficient to convert a desired amount of feedstock to synthesis gas or "syngas." Reaction temperatures typically range from about 900.degree. C. to about 2,000.degree. C., preferably from about 1,200.degree. C. to about 1,500.degree. C. Pressures typically range from about 1 to about 250, preferably from about 10 to about 200 atmospheres. The average residence time in the reaction zone generally ranges from about 0.5 to about 20, and normally from about 1 to about 10 seconds.
The syngas reaction product leaving the partial oxidation reactor generally includes CO, H.sub.2, steam, CO.sub.2, H.sub.2 S, COS, CH.sub.4, NH.sub.3, N.sub.2, volatile metals and inert gases such as argon. The specific product composition will vary depending upon the composition of the feedstock and the reaction conditions. Non-gaseous byproducts include particulate materials, generally carbon and inorganic ash, much of which is entrained in the product stream and carried out of the reactor. Some of the non-gaseous byproducts contact the inside surfaces of the reactor and adhere thereto as slag.
Slag is essentially fused mineral matter, such as ash, the byproduct of the slag-depositing material in the feedstock. Slag can also include carbonaceous materials, such as soot. Slag materials also include oxides and sulfides of transition metals such as vanadium, molybdenum, chromium, tungsten, manganese, and palladium, which can be recovered as valuable byproducts of the slag.
The molten slag that flows out of the reactor is generally collected in a quench chamber. Slag that accumulates in the quench chamber can be discharged periodically to slag trapping means, such as a lockhopper or other suitable vessel.
Slag that has a higher melting point than the reactor temperature conditions generally builds up as solid deposits in the reactor, most often on the refractory surfaces lining the reactor. Slag deposits tend to increase as the gasification reaction proceeds, and can build up to a level where removal or deslagging becomes desirable or necessary.
When the need for slag removal arises, the gasification reaction is stopped and "controlled oxidation" or deslagging commences. Controlled oxidation conditions in the reactor are designed to melt out and remove the accumulated slag.
Deslagging is also warranted when slag buildup occurs in the quench chamber. Such slag buildup in the quench chamber can cause premature shutdown of the partial oxidation reactor since the slag can fill the quench chamber and restrict the gas path to the throat of the reactor.
The slag is generally physically removed, such as by chipping it away from the refractory surfaces and/or by drilling it out from the openings or passages that have become partially or completely blocked by the slag. Needless to say, such methods of slag removal can damage the reactor and must be conducted very carefully.
To obtain maximum deslagging rates, the gasifier temperature during controlled oxidation should operate at a temperature of about 1000.degree. C. to 1500.degree. C. and preferably about 1100.degree. C. to 1400.degree. C.
During the controlled oxidation reaction, the partial pressure of oxygen is increased in the gasifier to convert the high melting temperature V.sub.2 O.sub.3 phase into the lower melting temperature V.sub.2 O.sub.5 phase. Any free-oxygen-containing gas that contains oxygen in a form suitable for reaction during the partial oxidation process can be used. Typical free-oxygen-containing gases include one or more of the following: air; oxygen-enriched air, meaning air having greater than 21 mole percent oxygen; substantially pure oxygen, meaning greater than 95 mole percent oxygen; and other suitable gas. Commonly, the free-oxygen-containing gas contains oxygen plus other gases derived from the air from which oxygen was prepared, such as nitrogen, argon or other inert gases.
The partial pressure of oxygen is generally gradually increased during controlled oxidation from about 1.0% to about 10% at a pressure of about 10-200 atmospheres in the partial oxidation reactor over a period of about 2 to 24 hours.
Various means for the detection and monitoring of slag accumulation in the reactor or its outlet have been attempted. The monitoring of slag buildup is important to determine when deslagging is needed and thereby anticipate the need for deslagging in advance of reactor shutdown. It is also important to monitor slag removal during deslagging or controlled oxidation to measure the progress and completion of the deslagging operation.
Slag deposits can be visually observed by means of a borescope mounted in the reactor opening and positioned to provide a view of the reactor walls or outlet. Visual observation can also be made with fiber optics sited by the burner to detect light radiating from the slag or refractory in the reactor outlet or other area. Nuclear or sonar detection can also be used to measure variations in slag thickness.
The use of thermocouples mounted in different reactor locations can provide information about variations in temperature measurements, that is, a temperature profile along the reactor walls to enable the detection of accumulating slag deposits.
Pressure change in the reactor has also been measured to monitor the presence of slag deposits, since increasing slag deposits in the reactor outlet can constrict gas flow through the outlet and build up measurable pressure within the reactor. Correspondingly, pressure drops in the reactor can indicate a clearing of slag deposits that obstruct the reactor outlet.
Despite the availability of known methods for monitoring slag buildup in the reactor, a major drawback of these methods is their degree of difficulty and cost to monitor the progress and completion of slag removal in the partial oxidation reactor during controlled oxidation.